1. Technical Field
This invention relates generally to heat exchangers and, more particularly, to a heat rejecting refrigerant-to-air finned coil heat exchanger used as a condenser in refrigeration and air conditioning devices.
2. Background Art
Heat transfer is a function of available temperature difference and of time. The larger the temperature difference, the faster the heat transfer. However, for the same degree of available temperature difference, heat transfer can be increased by allowing longer real time contact between the two heat exchanging media. Complete heat transfer can be assured at all times by allowing an appropriate duration that heat exchanging media stay in contact.
In the prior art, finned-coil heat exchangers using forced air are common. These exchangers always approached the shape of a slab, i.e., a large surface area with a very thin depth. This xe2x80x9cslabxe2x80x9d is often bent to form a xe2x80x9cU-shapexe2x80x9d. Generally, the length dimension or width dimension or both of the coil surface area is many times greater than the dimension of the depth of the coil, 4 to 20 times or higher. This decreases resistance to the air movement, but enormously reduces the actual time during which cooler air is in contact with the hotter refrigerant tube surface. The short real time contact between the air and the fins results in a much smaller temperature rise being imparted to the cooler air passing over the fins.
In a typical refrigerant cycle, the usual available temperature difference is about 30xc2x0 F. including the superheat. This is the difference between the temperature of the hot refrigerant fluid entering the heat exchanger and the temperature of the same fluid leaving the exchanger. However, the temperature rise of the air passing over the fins through the heat exchanger is typically only about 10xc2x0 F., about one-third of the maximum available. That means about three times as much air is being moved as the minimum needed. The larger air quantity being moved means that more energy is being expended to move air.
Another problem with prior art finned-coil heat exchangers is that the general xe2x80x9cslabxe2x80x9d shape necessitates larger overall volume of the unit. It therefore has a larger footprint, so it occupies more floor space. Since about three times more air is moved than needed, the unit becomes noisier. Additionally, with a large surface area of the coil relative to the sweep of the fan blades, uneven air flow over the coil is created. Because of this, excessive amounts of air pass through the coil surface that is closest to the fan, while the peripheral areas of the coil are starved. That is, no air is moved over the coil portions radially remote from the fan center. This fact means that the full heat transfer capacity of the coil is not being utilized.
In the prior art, the fin density is very high. Typically, heat rejecting condensers use a minimum of 10 fins per inch with 12 to 14 fins per inch being common and 16 fins per inch being the upper limit. The spacing between tubes carrying the fins also has a typical dimension. For instance, the distance between the center lines of tubes having an diameter of xe2x85x9c inch is a maximum of 1 inch; xc2xd-inch tubes, 1.25 inches; and, ⅝-inch tubes, 1.5 inches. In other words, the maximum air space between these tubes is 0.625 inch, 0.750 inch, and 0.875 inch, respectively.
Attempts have been made in the past to increase the air path by moving air along the longer dimension of the cross section of a finned coil. Andreoli U.S. Pat. No. 3,470,947 shows a convector radiator with a monobloc housing wherein ambient air enters from an open bottom, rises through tube fins and exits from the radiator at its upper front corner. Drewes Canada Patent No. 591,553 discloses fins having a large vertical dimension and a smaller depth dimension. Monroe U.S. Pat. No. 3,867,981 employs an angularly sloped flanged fin wherein air is moved across the longer dimension to generate greater heat exchange. While air is moved across the longer side of the fin, no blower is provided to increase air flow. None of these patents show counterflow between the two media needed for efficient heat transfer. These patents also do not show the use of a large number of tube paths needed to purposely create a longer air path.
Umehashi Japan Patent No. 56-3834 shows air path partially along the longer dimension of the finned cross section in an heat absorbing evaporator unit of an air conditioner. Umehashi does not show tubes having a large number of segments transversing air flow necessary to obtain a long air path and good countercurrent (or counterflow) effects. Kormso et al. U.S. Pat. No. 4,483,392 shows air drawn across rows of tubes only three deep. Neither Umehashi nor Kormso show counterflow effects.
Kritzer U.S. Pat. No. 3,151,671 shows a laterally situated blower with air moving along the longer dimension of the finned cross section of a heat radiator employed for comfort heating of indoor space. Kritzer does not show the utilization of transversely spaced multiple tubes to achieve longer path. In heat radiators used for indoor comfort heating applications, it is not essential that complete heat exchange take place by dissipating all heat available in the fluid to the space. In fact, in indoor comfort heating applications, the heat dissipated always varies and gradually lessens as room temperature approaches the thermostat setting. There is nearly always less than complete heat exchange.
Yanadori et al. U.S. Pat. No. 4,333,520 shows air moving along the longer dimension of the finned cross section of an indoor air conditioner unit. Yanadori et al. does not show need for multiple tube paths in an aligned row to obtain long air path for complete heat transfer with minimum air movement and does not show the two mediaxe2x80x94air and fluid in the tubesxe2x80x94flowing in counterflow directions.
As stated above, it is not essential that there be complete heat transfer between air and the fluid in the tubes of an indoor space heating unit or space cooling unit. For a heat rejecting refrigerant-to-air condenser to be efficient, it is only essential that all heat available in the refrigerant fluid with respect to the ambient temperature be rejected. In an indoor application, it is not desirable that heat transfer take place with minimum air movement. Minimum air movement can cause uncomfortably cold air to emanate from the unit or extremely hot air to blow out of the unit. In extreme situations, this can cause icing of the coil in a cooling mode or a fire hazard in a heating mode. In an indoor space heating unit or space cooling unit application, the heat transfer between the air and the fluid within the tubes continuously varies. It gradually decreases as the space being conditioned approaches the thermostat temperature setting. Air needed to deliver heat or cooling to a distant point in the room must have a small temperature difference from ambient. It can neither be too cold nor too warm so as to become uncomfortable. Both these considerations require that high volumes of air be moved. Yet, complete heat transfer from the tube media should be obtained for maximum efficiency.
The present invention is directed to overcoming one or more of the problems as set forth above.
According to the present invention, a heat exchanger is provided with a housing having spaced front and back walls and spaced side walls defining an internal chamber area with open lower and upper ends, tubes for conducting hot fluid into the chamber area, a series of spaced fins in contact with the tubes to transfer heat from the tubes to air within the chamber area, the tube segments being spaced transverse to the air path between the two openings located at the opposite ends of the chamber area. The tubes are arranged in rows which are substantially parallel to air flow path with the hot fluid entering the chamber at the air outlet end and traveling through the rows in a counterflow direction with the air so that the multiple tube segments provide a longer air path allowing for longer real time contact between cool air and the hot fins. As a result, more time for heat transfer with minimum air flow is achieved with a reduction in energy consumption.
In an exemplary embodiment of the invention, the housing has a rectilinear configuration, the tubes are routed back and forth through the fins which are oriented parallel to the air flow from a high upstream end to a low downstream end. The fluid flow direction is parallel but opposite to the air flow direction to provide a counterflow effect.
In a preferred embodiment of the invention, each tube is routed back and forth at least 6 times so that at least 6 segments of the tube are connected with air flow across the segments providing a counterflow effect. The tubes and their respective tube segments are spaced apart sufficiently to minimize their resistance to air flow. Further, the fins are spaced apart at a density not exceeding 8 fins per inch to minimize fin resistance to air flow.
A feature of the invention is that air resistance due to longer flow path is reduced by increasing tube spacing. It is noted that for tubes having a diameter not exceeding xc2xe inch the spacing between adjacent tube centers should be twice the tube diameter plus at least xc2xd inch. Increased tube spacing has the advantage of providing a larger fin area per tube with a lesser number of fins. For example, if tube spacing were increased from 1 inch to 2 inches, the available fin area per tube would increase from 1 inch by 1 inch (1 square inch) to 2 inches by 2 inches (4 square inches). By doubling the tube spacing, the fin surface area available per fin per tube would be increased 4 times. Therefore, by doubling the tube spacing, the fins per square inch can be reduced by a factor of 4. Air resistance is drastically reduced by simply increasing tube spacing and reducing fin density. Each of these changes reduces the physical obstruction to air flow and, together, provide for an even greater reduction in obstruction to air flow.
As a general rule, if tube spacing is increased, air resistance is reduced. Experimental tests would indicate that in a heat exchanger having at least 6 tube rows, if the tube spacing were increased to twice the tube diameter plus at least xc2xd inch but less than xc2xe inch, fin densities greater than 8 fins per inch increase air resistance to levels where no advantage can be obtained. However, in the same heat exchanger, if the tube spacing were increased to twice the tube diameter plus at least xc2xe inch or more, air resistance is reduced so that fin density can be increased above 8 fins per inch and still provide the benefit of reduced power and increased heat transfer.
An objective of this invention is to alleviate the above mentioned problems associated with prior art fin-tube heat exchangers, namely, higher energy usage due to excessive amounts of air moved, larger overall unit volume, uneven air flow through the exchanger, larger xe2x80x9cfootprintsxe2x80x9d, and higher levels of noise.
By maintaining longer contact between the cooler air and the hotter fins, most of the heat of the fins can be transferred with a minimum of air flow. A longer path for the air can be achieved by making the air pass over a number of segments of the same fluid containing tube. The increased resistance to the air due to multiple tubes is moderated by the use of less fins or by decreasing their density. The use of a longer air path over a large number of tube bends combined with the principle of complete counterflow and small fin density reduces the energy needed for operation. Further, reducing the face area of the heat exchanger coil relative to the physical size reduces uneven air flow through the coil which would otherwise result in a loss of heat transfer capacity of the heat exchanger.